In general, dot matrix printers are constructed with a moveable printhead. The printhead includes a nosepiece with a matrix of printhead pins which are actuated at timed intervals to print. The nosepiece operates in combination with a loop of inked ribbon to imprint paper which is fed across a printing platen. For high speed and multi color printing a gear driven ribbon guide is utilized to shift the ribbon to a selected position with respect to the nosepiece of the printhead. This permits parallel stripes or segments of different colors along the width of the ribbon to be selectively aligned with the nosepiece for multi color printing. Alternately, if the ribbon is of a solid color the ribbon guide permits bands along the entire width of the ribbon rather than just a single band to be utilized in printing. The ink supply of the entire ribbon can thus be depleted. The coordination and timing of the printhead, nosepiece, ribbon and ribbon guide are controlled by a microcomputer.
Typically, the ribbon guide is pivotally mounted to the printhead and is coupled through gear teeth to a motor driven gear. Rotation of the gear through a small angle rotates the ribbon guide through a corresponding angle and displaces the ribbon with respect to the printhead to one of a selected number of positions.
The ribbon guide is typically formed with an opening adjacent to the printhead and the ribbon is threaded through the ribbon guide and across this opening. Support segments of the ribbon guide formed on either side of the opening adjacent to the printhead, support the ribbon for contact with the printhead and the printing paper.
U.S. Pat. No. 4,568,209 to Zerillo discloses a ribbon cartridge assembly for dot matrix printers having such a ribbon guide.
One problem with this type of ribbon guide is that the ribbon is generally formed as a closed or endless loop and must be threaded through the ribbon guide upon initial manufacture of the ribbon cartridge. Accordingly, there is no provisions for field replacement of just the ribbon guide portion of the ribbon cartridge as might be required with a worn ribbon guide.
Another problem with this type of ribbon guide is that the action of the ribbon guide may distort the printing. In particular, the curved outer surface of the ribbon guide as well as the curved support segments for the ribbon, press the paper into a corresponding curved shape. Consequently the printing may be slightly distorted by interaction with the curvature of the paper.
The present invention is directed to a ribbon guide that overcomes these limitations. The ribbon guide of the invention is constructed with open slits for releasably securing the ribbon to the ribbon guide. This allows a ribbon guide to be mounted to or removed from a printhead and a ribbon secured to or released from a ribbon guide without breaking the closed loop of ribbon. A worn ribbon guide can thus be replaced in the field or a ribbon cartridge carrying either a closed loop or double ended ribbon may be replaced without removing the ribbon guide.
Additionally, the ribbon guide of the present invention is constructed with faceted rather than curved support segments for the ribbon and a faceted outer surface for contact with the paper. The flat faceted segments help maintain the ribbon and paper parallel to the printing platen so that the printhead pins may strike the print at a right angle to the ribbon and paper.